Buck converter LED drive systems for automotive applications

In this second of a series of articles on power converters, Iain Mosely describes how the Buck converter switching topology can be used to drive LEDs efficiently in demanding automotive applications.

Oct 17th, 2007
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Electronic systems operating in the automotive environment need to withstand high ambient temperatures and high-energy voltage transients on the power source. Load-dump situations can result in around 60V or more being present transiently on a 12V power rail.

The example LED system presented in this article drives two Luxeon K2 LEDs at 1A from an input voltage of 10V to 65V with a nominal efficiency of 87%.

The generic Buck converter power stage is shown in Figure 1(a). When switch SW1 turns on, the voltage on node Vs is equal to the input voltage. When SW1 turns off, the inductive current flowing in L1 forces D1 to conduct and clamps the voltage on node Vs to be very close to zero. The resulting Vs node voltage waveform is a high frequency square wave as shown in Figure 1(b).

Inductor L1 and capacitor C2 form a low pass filter to recapture the DC (or average) value of the switching waveform. Whenever SW1 conducts current, it has zero voltage across it and when it blocks current, it has high voltage across it. This results in a highly efficient power-conversion process.

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This article was published in the September/October 2007 issue of LEDs Magazine.

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